Development of Catalytically Functionalized Polyester-Based Filters Produced by Flame Spray Pyrolysis

Abstract

For industrial processes—like waste incineration—it is necessary to reduce solid components (like dust or fly ash) as well as gaseous components (like dioxins, CO and other harmful hydrocarbons) to fulfill legal requirements. Therefore, catalytically functionalized filters based on polymers already exist. However, it is known that such filters are always constructed in multiple layers to prevent the migration of catalyst particles. This study demonstrates that it is possible to prepare a stable catalytic functionalized single-layer filter based on polyester needle felt by using flame spray pyrolysis. The catalyst is a low temperature active Pt/TiO2 with a loading weight of 38 g/l on the filter. Via SEM images the uniform distribution of the catalytic particles even in the deeper regions of the single-layer filter was proven. The structure was confirmed after experiments under realistic conditions—migration could not be obtained. Likewise, it was obtained that the oxidative conversion of carbon monoxide (CO) to carbon dioxide (CO2) is completely even at temperatures below 100 °C. Furthermore, comparative studies with catalysts on a honeycomb and a ceramic foam have shown that the conversion on the polyester needle felt textile catalyst is comparable.

Full text

Vol.:(0123456789)
Topics in Catalysis (2024) 67:539–550
https://doi.org/10.1007/s11244-023-01892-7
ORIGINAL PAPER
Development ofCatalytically Functionalized Polyester‑Based Filters
Produced byFlame Spray Pyrolysis
D.Bissinger1· J.H.Honerkamp1· J.Roldan1· J.Bremes1· K.Kannen1· M.K.Lake1· A.Roppertz1
Accepted: 6 November 2023 / Published online: 6 January 2024
© The Author(s) 2024
Abstract
For industrial processes—like waste incineration—it is necessary to reduce solid components (like dust or fly ash) as well
as gaseous components (like dioxins, CO and other harmful hydrocarbons) to fulfill legal requirements. Therefore, catalyti-
cally functionalized filters based on polymers already exist. However, it is known that such filters are always constructed in
multiple layers to prevent the migration of catalyst particles. This study demonstrates that it is possible to prepare a stable
catalytic functionalized single-layer filter based on polyester needle felt by using flame spray pyrolysis. The catalyst is a
low temperature active Pt/TiO2 with a loading weight of 38g/l on the filter. Via SEM images the uniform distribution of the
catalytic particles even in the deeper regions of the single-layer filter was proven. The structure was confirmed after experi-
ments under realistic conditions—migration could not be obtained. Likewise, it was obtained that the oxidative conversion
of carbon monoxide (CO) to carbon dioxide (CO2) is completely even at temperatures below 100°C. Furthermore, compara-
tive studies with catalysts on a honeycomb and a ceramic foam have shown that the conversion on the polyester needle felt
textile catalyst is comparable.
Keywords Polyester textile· Oxidation catalyst· CO· Flame spray pyrolysis· Catalytic filter· Combined gas and particle
treatment
1 Introduction
Industries across the spectrum, from manufacturing to
chemical processing and transportation, invariably gener-
ate airborne and waterborne particles as byproducts of their
operations. These particles, often comprising hazardous sub-
stances, pose a dual threat: they contribute to air, degrading
the quality of our natural resources and compromising the
well-being of ecosystems and human populations. Moreover,
these emissions frequently contain greenhouse gases and
other pollutants like NOx, HC or SOx that exacerbate climate
change and contribute to the broader spectrum of environ-
mental challenges faced globally [1, 2]. Waste incineration
exemplarily is one of the main sources of industrial dust
and gas emissions. The exhaust gases contain high levels of
dust (> 10mg/m3) and harmful hydrocarbons (> 10mg/m3).
To remove the gaseous and solid emissions different setups
are in use and the temperature range at the removal point
varies between 100 and 300°C [3]. These conditions allow
to use of filters and separate catalysts. The filters used for
this purpose often rely on polymer or ceramic materials and
are characterizes by face velocities of 0.5 to 2 m3/m2·min.
Catalysts, on the other hand, consist almost exclusively of
metallic or ceramic substrates with varying cell density and
coating thickness. Depending on the specific application,
the thickness of the catalytic material layer ranges from 15
to 100µm, which can result in loading weights of 30 to
200g/liter, depending on the material. Cell densities typi-
cally ranges from 50 to 400 cpsi depending on the system
requirements. With increasing cell density, activity and
pressure drop usually also increase. As the pressure drop
is a critical factor for economic reasons, ceramic honey-
combs with a maximum of 233 cpsi are typically used in
industrial plants. Depending on the conditions of use, these
can be used as solid extrudates or as coated honeycombs,
typically using cordierite or Al2O3 [4]. Besides honeycomb
substrates, ceramic foams serve as another option for cata-
lyst supports. In the field of emission reduction in small
combustion plants, foams based on Al2O3 with a pore size in
* A. Roppertz
Andreas.roppertz@hs-niederrhein.de
1 University ofApplied Science Niederrhein, Adlerstrstr. 32,
47798Krefeld, Germany
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540 Topics in Catalysis (2024) 67:539–550
the range of 10–30 ppi are often the preferred choice. Due to
their irregular structure and spacious pores, these foams can
be utilized with low back pressure. Typical space velocities
for ceramic foams and honeycombs fall within the range
of 30,000 to 80,000 h−1. The type of oxidation catalyst in
the field of emission reduction often depends on the operat-
ing temperature. While precious metal-free oxide materials
such as MnO2, Fe2O3 or mixed oxides are commonly used at
temperatures above 200°C, precious metal-based catalysts
with platinum or palladium are often used for applications
below 200°C [4].
When using separate catalysts and filters to remove dust
and emissions, the respective pressure losses must be con-
sidered additively and may be detrimental to the operation
of the plant. Furthermore, separate units require a consider-
able amount of space. Against this backdrop, the concept
of simultaneous particle filtration and emissions reduction
has become a focal point in efforts to achieve sustainable
industrial practices. An example of this is the exhaust air
purification of waste incineration plants, which already relies
on this technology [5]. The type of filter medium depends
on many factors, with the process parameters temperature
and exhaust composition being particularly decisive. In
addition, the amount and size of the materials to be filtered
is, of course, crucial for the type of filter. Additionally, the
necessity of periodic filter cleaning through short pressure
shocks is pertinent. These cleaning cycles involve the injec-
tion of pressure shocks, causing a reverse flow to dislodge
filtered dust particles from the filter. These pressure shocks
impose a mechanical stress on the catalytic particles fixed
on the filter, emphasizing the critical need for their secure
attachment to the filter.
The use of ceramic filter cartridges has become estab-
lished in the exhaust air purification of many industrial
processes with exhaust gas temperatures above 300°C [6].
Generally, they are characterized by high filtration efficiency
of above 99.5% at broad temperature ranges [7, 8]. How-
ever, it is necessary to distinguish between different types
of ceramic filters. In addition to ceramic membrane filter
cartridges which are characterized by low porosity and high
filter performance, there are also ceramic fiber filters exist-
ing with a high porosity and lower filter performance. The
latter is also suitable for catalytic coating, as the catalyst
particle do not clog the huge channels and thus not increase
the backpressure [9–18]. Those ceramic materials can be
coated be several procedures like dip coating or incipient
wetness coating, e.g. ceramic fibers coated with SCR cata-
lysts are widely used for simultaneous particle filtration and
DeNOx [19–21].
For applications at temperatures below 250°C also PTFE
filter candles can be used. They could also be catalytically
coated however the coating process is more complicated
as there are no attractive interactions between the PTFE
surface and the inorganic catalyst particles. Thus, the addi-
tion of binders is necessary which makes the preparation
more demanding. Typically, a bag over bag principle is used
consisting of at least two layers of PTFE filter. This has the
advantage that the dust particles are captured on the outer
layer while the emissions pass the second catalytically func-
tionalized filter on the inner filter bag [22]. However, multi-
layer systems naturally increase the pressure drop, which has
an unfavorable effect on the operating conditions. Moreover,
as the preparation contains binders, the catalytic particles are
deposited not only on the fibers but also in the interstices,
which also increases the pressure drop [23]. Finally, there
is the question of whether polyfluorinated materials such
as PTFE should be used for this purpose in the long term,
as their impact on the environment is considered to be of
concern [24].
For filtration tasks at temperatures below 150°C, pol-
yester needle felts are very popular. These materials are
inexpensive and are characterized by high filtration perfor-
mance. Depending on the filtration task, they can be used
in a wide range of basis weights ranging between 250 and
600g/m2 [25]. Catalytic functionalization of polyester nee-
dle felt textiles has not been considered so far, as polyester,
similar to PTFE, has no functional groups to which the inor-
ganic particles could be chemically bound. Consequently, a
binder-based process would also have to be used, but this
is not feasible due to the high cost and limited applicabil-
ity in terms of temperature stability. A process for direct
coating of textile fibers could overcome this problem and
make the catalytic functionalization of polyester materials
attractive. The flame spray pyrolysis (FSP) used in this paper
addresses this point by baking the catalytic particles into the
polyester needle felt textile fibers in a form closuring way.
FSP is typically used to prepare nanoparticles by contacting
a precursor solution with a flame that initiate the chemical
transformation [26]. Within this paper the FSP is used the
heat up the already prepared catalytic particles in order to
contact them with the polyester textile. This novel and suc-
cessful approach is described in detail below.
2 Experimental
2.1 Reagents
A solution of Bis(ethanolammonium)hexahydroxoplatinum(IV)
(Precious metal content 9.88 wt%) in liquid form was
acquired from Heraeus. The SiO2-doped TiO2, identified
as DT-S10, was purchased from Tronox. Carl Roth sup-
plied the solution of Cerium(III) nitrate hexahydrate. The
polyester needle felt (with a basis weight of 550g/m2) was
obtained from Kayser Filtertech. Hoffman Ceramic provided
the 20ppi Al2O3 foam. The 233 cells per square inch (cpsi)
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541Topics in Catalysis (2024) 67:539–550
Al2O3 honeycomb was supplied by Porzellanfabrik Herms-
dorf. Acetic acid was procured from Sigma Aldrich.
2.2 Catalyst Powders
2.2.1 Pt/TiO2
The catalyst was prepared via incipient wetness impreg-
nation. For this purpose, 5g of DT-S10 was placed in a
50 ml tumbler. Subsequently an aqueous solution of
bis(ethanolammonium)hexa-hydroxoplatinum(IV) was
slowly dropped onto the DT-S10 while stirring the solid.
The mixture was dried overnight at 80°C followed by a
calcination step at 400°C for 1h. The adjusted mass fraction
of platinum was set to 4 wt%, based on the DT-S10.
2.2.2 Pt/Ce/TiO2
Pt/Ce/TiO2 was synthesized following the same route as
described for Pt/TiO2. However, prior to the addition of plat-
inum, cerium oxide was impregnated by incipient wetness
impregnation. Therefore, an aqueous solution of cerium(III)
nitrate hexahydrate was used. The four resulting catalysts
contain between 1 and 10 wt% of cerium by mass. The
nomenclature of the catalysts is adjusted according to the
mass content and is therefore Pt/Ce2/TiO2 for the catalyst
loaded with 2 wt% cerium. The adjusted mass fraction of
platinum is again 4wt% based on DT-S10.
2.3 Coated Catalyst Materials
2.3.1 Flame Spray‑Based Synthesis ofPolyester Needle Felt
Textile Catalyst
To prepare the catalytic polyester needle felt textile, a
120mm round piece of polyester needle felt was used
in a self-designed FSP as shown in Fig.1 (left). The
FSP consists of a two-substance nozzle (Lechler, Model
136.316.16.A) with a spray angle of 25° and two commer-
cial gas burners. Furthermore, it consists of a black steel
tube with 120mm diameter and variable length between
400 and 500mm, in which the polyester needle felt was
fixed to the bottom. To suck the air containing particles
into the black tube, a compressor was connected under-
neath (SKV-NS-95-1-101) with an 80mm flexible pipe.
During preparation, the pressure above and below the pol-
yester needle felt was measured with a differential pres-
sure system. For synthesis, the polyester needle felt textile
was attached to the bottom of the tube and the compressor
was adjusted to maintain a constant differential pressure
of 30mbar. After starting the burners, a temperature of
approx. 130°C was reached on the polyester needle felt
textile, measured with two K-Type thermocouples placed
directly above the textile and in the middle of the black
tube. The Pt/TiO2 catalyst particles (solids content approx.
20 wt%) suspended in a solution of water and acetic acid
(2:1) are metered into the two- fluid nozzle by a peristaltic
pump (ProMinent Type Dulcoflex DF4a). The dosing rate
Fig. 1 Flame spray apparatus
(FSP) for coating of permeable
polyester needle felt textiles
(left). Uncoated (top) and
coated (bottom) sample in FSP
(right)
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542 Topics in Catalysis (2024) 67:539–550
of the stable suspension was 0.00143l/min while the air
flow rate was 10l/min. When the atomized suspension
passes the burner the catalytic particles immediately heat
up before they impact the polyester needle felt textile. As
the particle temperature as well as the polyester needle
felt textile temperature was above the glass transition tem-
perature of the polyester needle felt (at around 75°C) the
particle can easily penetrate the textile fiber. As shown
in Fig.1 (right) the particle deposition exclusively takes
place in the area where the compressor sucks air through
the polyester needle felt. The dosing time of the suspen-
sion was adjusted so that the loading weight with catalyst
reaches almost 38g catalyst per liter of substrate (textile),
what was also confirmed gravimetrically and is within
the typical loading range for oxidation catalysts, which
is often between 30 and 80g/l. It has also been observed
that this coating method is highly reproducible at a loading
weight of approx. 38g/l.
2.3.2 Coated Ceramic Catalysts
The Pt/TiO2 catalyst was coated on both honeycomb (233
cpsi) and foam (20 ppi). The Pt/Ce/TiO2 was exclusively
coated on a 20 ppi ceramic foam.
2.3.2.1 Preparation of Catalyst Solution For coating the
ceramic carriers, the powdered Pt/TiO2 catalyst was trans-
ferred to an aqueous suspension (Solid content between 30
und 40 wt%). First, pH was adjusted to 4 by using diluted
HNO3 before adding 1 wt% of a powdered boehmite (Pural,
Fa. Sasol) while stirring. In addition, a small amount of
methyl cellulose was added to enhance viscosity. The solids
content was adjusted so that the target amount of solid could
be achieved with a single dip coating process. A catalyst
loading of 38g dry mass per liter substrate volume was the
targeted value.
2.3.2.2 Foam Catalyst A cylindrical Al2O3 foam ceramic
(20 ppi) with 25.4 mm diameter and a length of 12mm
is coated by a dip coating process. First, the substrate was
dipped into the liquid catalyst solution for several seconds.
Subsequently it is carefully blown out with air. The so pre-
pared ceramic was then dried at 80 °C overnight before
calcination at 400°C for two hours in air atmosphere. As
previously mentioned, the Pt/Ce(2 to 10)/TiO2 materials are
also prepared on foams. For preparation the same synthesis
route was used.
2.3.2.3 Honeycomb Catalyst A cylindrical Al2O3 honey-
comb (233 cpsi) with dimensions 25.4mm (Diameter) and
a length of 12mm was prepared by dip coating. The proce-
dure used was the same as described for the ceramic foam.
3 Characterization
3.1 Powder Catalysts
3.1.1 Nitrogen Sorption Experiments
Nitrogen sorption experiments (N2) were conducted on a
Surfer (Thermo Fisher Scientific, Waltham, MA, USA) at
a temperature of −196°C. Data evaluation was done using
the Thermo Fisher Scientific Software Surfer Version 1.7.2).
BET-plots were acquired a relative pressure range from 0.05
to 0.3.
3.1.2 X ray Diffraction
The X ray diffraction analysis was performed on a D2
PHASER 2nd Gen from Bruker. The X-ray source is a Cu
anode with a wavelength of λ(Kα) = 1.52Å and an accelerat-
ing voltage of 30kV at 10mA. The primary optic was a 4°
Soller aperture with a 2mm divergence slit. The secondary
optic was a 4° Soller aperture with a nickel filter. The detec-
tor was a LEE-T sensor with 192 channels and a resolution
below 380eV (Cu).
3.1.3 Transmission Electron Microscopy
The Transmission electron microscopic measurements were
performed on a Jeol JEM-2100 Plus transmission electron
microscope at an applied accelerating voltage of 200kV. For
the preparation, the sample was first mortared in ethanol for
one minute. Then the suspension was sedimented for another
10min and a small amount was taken up from the top layer
using a pipette. A small drop of the absorbed volume was
consumed on the holey carbon layer of the copper TEM
grid. After a short time, the ethanol was volatilized and the
sample carrier including the sample can be placed in the
measuring instrument.
3.2 Substrate Based Catalysts
3.2.1 Scanning Electron Microscopy
SEM images were taken with a Zeiss Gemini DSM 982
equipped with an Inlens detector. Prior to the measurements
the samples were sputtered with a 20nm carbon film in order
to enhance the electric conductivity.
3.2.2 Pressure Drop Analysis
The pressure drop was assessed within a custom-built test
bench comprising a 40mm inner diameter tube. A sample
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543Topics in Catalysis (2024) 67:539–550
could be attached to the end of the tube. For pressure meas-
urement, a differential pressure sensor was connected to the
tube upstream of the sample, while a second sensor operated
against ambient pressure. The pressure drop was recorded
across various flow rates ranging from 0 to 30l/min. The
sample dimensions were fixed at a 20mm diameter. The
effects of wall friction can be neglected under the specified
parameters.
3.2.3 Migration Tests Procedure
The same setup (as in 3.2.2.) is used to analyze the particle
adhesion on the polyester needle felt textile. The polyester
needle felt textile is clamped into the apparatus, and pressure
pulses (1s duration) are generated using an automated valve,
resulting in a differential pressure of 50mbar upstream and
downstream of the filter. The textile subjected to 1000 pres-
sure pulses during the test. The proportion of migrated par-
ticles is determined by gravimetric measurements before and
after the experiment.
3.2.4 Catalytic Activity inCO Oxidation
The catalytic activity of the polyester needle felt textile sam-
ples was tested in a different test cell than the other samples,
as described below. The results are shown as temperature-
dependent conversion.
Conversion was given as: XCO =1−
c
CO
CCO
,0
. The initial CO
concentration CO, 0 is given to 100ppm.
The coated polyester needle felt was measured in a self-
constructed cell consisting of two parts (Fig.2). The 20mm
round piece of polyester needle felt textile was clamped
between the two halves. It was fixed with O-rings (NBR).
A type K thermocouple was mounted under the polyester
needle felt textile to measure the inlet gas temperature. The
gas enters horizontally at the bottom of the cell in order
to ensure a uniform flow through the polyester needle felt
textile. The cell was electrically heated. Prior to the meas-
urement the sample was pre-treated for 30min at 150°C in
air. Subsequently the sample was cooled to 100°C and the
gas mixture was initiated. Data points were collected from
the cooling curve. The space velocity was set to 25,000 h−1,
50,000 h−1 and 75,000 h−1, with the gas matrix consisting
of 100ppm CO, 16 vol% O2, 2 vol% H2O by volume, and
N2 as balance. The flow rate of each gas was adjusted via
independent mass flow controllers (Bronkhorst® Germany
North,). Gas concentrations were measured at the reac-
tor outlet using an electrochemical sensor (Testo 340, Fa.
Testo).
3.2.5 Foam andHoneycomb
For the CO oxidation activity measurements, the respec-
tive samples were placed in a tubular quartz reactor with an
inner diameter of 30mm and sealed with quartz wool. The
reactor was placed in a horizontal tube furnace, connected
to the exhaust manifold, and heated up to 150°C in air for
30min. After the short pre-treatment the sample were cooled
to 100°C and gas mixture was initiated. Data points were
collected from the cooling curve. The space velocity was
set to 25,000 h−1, 50,000 h−1 and 75,000 h−1, with the gas
matrix consisting of 100ppm CO, 16 vol% O2, 2 vol% H2O
by volume, and N2 as balance. The flow rate of each gas was
adjusted via mass flow controllers (Bronkhorst® Germany
North,). Gas concentrations were measured at the reactor
Fig. 2 Test cell for catalytic measurements with the polyester needle felt textile. Left: Both halves of the measuring cell in which the textile is
inserted via seals. Right: Positioning the textile in the measuring cell on the inner gasket
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544 Topics in Catalysis (2024) 67:539–550
outlet using an NDIR spectrometer (Saxon Junkalor Type
Infralyt 80).
4 Results
4.1 Powder Catalysts
The specific surface area (SSA) of the powdered materi-
als, determined by N2 physisorption, provides values of
110 m2/g for the pure DT-S10 (TiO2) and 101 m2/g for the
Pt/TiO2 catalyst. The decrease in SSA is more pronounced
for the ceria containing catalysts. Samples with 1 and 2 wt%
cerium have an SSA of 85 m2/g, while samples with 5 and
10 wt% have a SSA of 82 m2/g and 68 m2/g respectively. The
reduction in specific surface area (SSA) could be attributed
to blockage of pores caused by ceria particles.
X-ray analysis shows reflexes (2θ) at angles of 25.3°,
37.60°, 40.5°, 49.8°, 54.36° and 62.70° for all samples
attributed to anatase phase of TiO2 according to JCPDS
with reference number 23-0278. Anatase phase is marked
as empty dots in Fig.3. In addition, a broad reflection for
the platinum (dotted grey) is found at 46.2° (2θ). Using this
signal from the Pt/TiO2 sample, the estimated average crys-
tallite size for the platinum is 5.2nm. The reflex attributed
to platinum is also found in the ceria loaded samples where-
fore similar crystallite sizes between 3.5 and 5.2nm can be
estimated. Reflexes at 28.43° and 32.98° (2θ) are attributed
to CeO2 according to JCPDS card 34-0394 marked as black
dots. These reflexes can exclusively be found for the sam-
ples with 5 and 10 wt% ceria content. Calculation of the
CeO2 crystallite size for the Pt/Ce10/TiO2 sample is 4.3nm.
This suggests that the estimated size of the cerium oxide
and platinum crystallites is evidently in the same range. The
found crystallite sizes are in good agreement with literature
data for Pt/CeO and Pt/TiO2 catalysts [27, 28]
TEM analysis is presented in Fig.4 for the sample con-
taining 10 wt% cerium. Small entities, measuring between 3
and 14nm in size, are observable, although precise attribu-
tion to either platinum or ceria is not possible. To estimate an
average particle size, several hundred particles were counted
and analyzed. Despite the broad particle size distribution,
Fig. 3 X ray diffractogram of the powder catalysts Pt/TiO2 (a), Pt/Ce1/TiO2 (b), Pt/Ce2/TiO2 (c), Pt/Ce5/TiO2 (d), Pt/Ce10/TiO2 (e). The identi-
fied signals can be assigned to TiO2 in the anatase phase (○) as well as to cerium oxide (●) and platinum ( )
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545Topics in Catalysis (2024) 67:539–550
an average particle size of 5.9nm can be computed, which
aligns well with the findings obtained from X-ray diffrac-
tion. As previously mentioned, these findings align with data
found in literature on Pt/TiO2 or Pt/CeO2 catalysts, known
for their low-temperature oxidation activity.
4.2 Coated Catalysts
To assess the catalytic activity of the coated polyester nee-
dle felt textile in comparison to conventional catalysts, sub-
strates comprising a honeycomb and a ceramic foam were
also coated. A coating weight of 38g/l was targeted for all
samples. The effective loadings are shown in Table1.
The as-prepared polyester needle felt textile was ana-
lyzed using scanning electron microscopy, revealing cata-
lytic particles uniformly deposited on the polyester fibers.
(Fig.5a, b). Moreover, the coating does not form a closed
catalytic layer, but is deposited evenly and sporadically on
the fiber, without large agglomerates. This is consistent
with theoretical calculations of the layer thickness, which
showed that due to the enormously high geometric surface
of the textile sample (62.3 m2 per liter of substrate), a layer
in the sub-micrometer range would result. As can be seen in
Fig.5a, no large particle agglomerates are observed in the
areas between the fibers. It can therefore be assumed that the
air permeability of the coated polyester needle felt textile
is not significantly influenced by the coating. The coating
can even be observed in the deepest layers of the textile
(downstream side), but with slightly decreasing intensity.
This result is surprising considering that the process is based
on a one-sided coating on the upstream side of the polyester
needle-felt. Fortunately, however, this means that coating
from both sides is not necessary, which is advantageous from
an economic point of view.
As shown in Fig.5c, the coating of the ceramic foam
results in a thin layer with a variable thickness between 20
and 120µm. The uneven coating thickness is a consequence
Fig. 4 TEM image of the Pt/Ce10/TiO2 catalyst (a). Exemplary measurment and visualisation of the particle size of Pt/Ce10/TiO2 in order to
create a particle size distribution (b)
Table 1 Summary of technical relevant parameters for the three cata-
lyst types
Dimension Textile Honeycomb Foam
Cell density n.a 233 cpsi 20 ppi
Specific Surface area (m2/l) 62.3 2.2 3.6
Effective catalyst loading (g/l) 38.4 39.2 37.7
Calculated layer thickness (μm) < 1µm;
nonclosed
shape
18 11
Pressure drop @ 10l/min
(mbar/m) 3 0.1 0.1
Pressure drop @ 20l/min
(mbar/m) 7.2 0.2 0.3
Pressure drop @ 30l/min
(mbar/m) 12.4 0.3 0.6
Face velocity @10l/min (m/s) 0.32
Face velocity @20l/min (m/s) 0.63
Face velocity @30l/min (m/s) 0.95
Filtration efficiency for PM 10
(%) < 99 n.a n.a
Migration@ 1000 pres. shocks
(%) < 1%
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546 Topics in Catalysis (2024) 67:539–550
of the process in which the aqueous solution is absorbed into
the substrate by dip coating and subsequently dried. With
these highly curved porous materials, capillary effects occur
during the drying process, resulting in an uneven deposition.
The coating on the honeycomb yields a continuous layer,
albeit with varying thicknesses as shown in Fig.5d. Here,
too, the corners are significantly more heavily coated than
the flat surfaces due to capillary effects. Considering the
relatively small geometric surface of the ceramic honeycomb
and ceramic foam (2.2 m2/l and 3.6 m2/l, respectively), the
determined coating thicknesses agree with the theoretically
calculated average values of 18 and 11µm, respectively.
4.3 Migration Tests
In addition to the uniform distribution of the catalytic
particles on the polyester needle felt, it is crucial for the
technical application that the particles adhere to the textile
fibers even in the case of process-related pressure surges
and do not begin to migrate. The aim of the migration tests
was therefore to evaluate the adhesion properties of the
Fig. 5 Electron microscopic images of the coated textile (a, b), foam (c) and the honeycomb (d)
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547Topics in Catalysis (2024) 67:539–550
catalytic particles on the polyester needle felt under real-
istic conditions. For this purpose, the polyester needle felt
is subjected to a treatment involving 1000 pressure shocks,
which resulted in a mass loss of less than 1% relative to the
coating mass. An additional 1000 pressure shocks did not
result in a further reduction of particle mass. Evidently,
the adhesion of the particles to the polyester needle felt is
robust enough to withstand the applied pressure shocks.
4.4 Pressure Drop
As previously stated, due to the coating methods, it can be
assumed that the catalytic particles are deposited exclu-
sively on the textile fibers and have only a minor impact
on the air permeability. To investigate this, pressure drop
experiments were carried out with both the uncoated and
the coated polyester needle felt. In addition, the pressure
drop of the ceramic catalysts was also tested. Due to the
different thickness of the test samples, the pressure drop is
presented as a normalized pressure drop related to the same
substrate length.
The choice of air volume in these experiments is based
on the operating conditions for polyester-based textile filters.
In this context, the face velocity is used instead of the space
velocity as defined below. The face velocity is calculated by
dividing the volumetric flow
V
per minute by the filter area
A, as shown here:
Typically, this value ranges from 0.5 to 2m/min. In the
experiments shown here, the face velocity is investigated in
the range between 0.32 and 0.95, which corresponds to a
volume flow of 10 to 30l/min as shown in Table1.
As depicted in Fig.6, the pressure drop for the ceramic
honeycomb and ceramic foam is notably low under the con-
ditions investigated, as expected. The investigation of the
uncoated and coated textile reveals an almost linear relation-
ship between pressure drop and face velocity in the observed
range. At a flow rate of 30l/min, the normalized backpres-
sure of the coated polyester needle felt textile is 12.1mbar,
while that of the uncoated is only 8.4mbar. However, the
effective difference in pressure drop between the uncoated
and coated polyester needle felt textiles also decreases with
decreasing face velocity. Since a structural change of the
textile due to the coating process could not be observed with
any analytical method, it can be assumed that the pressure
difference is due to the catalytic coating.
4.5 Catalytic Activity Tests
The catalytic activity of the polyester needle felt filter is
examined on the basis of CO oxidation to CO2. The oxida-
tion of carbon monoxide is an important reaction in almost
all technical combustion processes that generate particulate
matter. Three different space velocities were chosen for
the experiments. Converting these space velocities to face
Face velocity
=

V
A
[
m3
m2∙min
=m
min
]
Fig. 6 Pressure drop of the honeycomb (Δ) and ceramic foam (▲) as
well as the coated (■) and uncoated (●) polyester needle felt
Fig. 7 Catalytic activity of the coated polyester needle felt (a), honeycomb (b) and ceramic foam (c) at space velocity of 25.000 h−1 (●),
50.000 h−1 ( ) and 75.000 h−1 (○). Conditions: 100 vppm CO, 16 vol% O2, 2 vol% H2O with N2 balance
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548 Topics in Catalysis (2024) 67:539–550
velocities shows that this range covers the typical face veloc-
ity for filters, roughly from 0.5 to 2 m3/m2min.
Regarding Fig.7 it can be stated that all catalysts exhibit
high activity at the space velocities and temperatures
selected, confirming the low-temperature activity of the Pt/
TiO2 catalyst. High CO conversions in oxygen-rich environ-
ments at temperatures below 100°C are already documented
in the literature for Pt/TiO2 catalysts [29]. When comparing
the catalysts investigated here, it can be observed that the
catalytically functionalized polyester needle felt provides the
highest activity (Fig.7a). This material initiates CO conver-
sion at 40°C. At the lowest space velocity, complete CO
conversion is already evident at 70°C. Even at higher space
velocities, complete conversion is observed from 90°C
onwards. Thus, the temperature range is well-suited for the
mentioned applications regarding simultaneous filtration and
emission reduction. Looking at the CO conversion on the
honeycomb catalyst (Fig.7b), it is evident that although the
conversion starts at lower temperatures, it follows a different
trend with increasing temperature compared to the textile,
and complete CO conversion cannot be achieved within the
temperature range independent of the space velocity. A very
similar activity trend can be observed for the ceramic foam
(Fig.7c). Here, a noticeable CO conversion is evident at
30°C, irrespective of the space velocity. However, complete
conversion cannot be achieved for this catalyst at any space
velocity. Yet, with the lowest space velocity of 25,000 h−1,
an almost complete (98%) CO conversion is attained at
100°C. It is worth noting that the superior activity of the
polyester needle felt catalyst is not necessarily attributable
to the substrate structure and the resulting low coating thick-
ness. Instead, it is presumed that the catalytic Pt/TiO2 par-
ticles undergo a chemical reduction during the flame spray
coating process, a phenomenon well described in the litera-
ture. [30]. Often, pre-reduction of a catalyst increases its cat-
alytic activity. For this reason, a pre-conditioning at 150°C
in air for 30min was conducted, which, however, might not
have been sufficient to establish a comparable state of the
catalyst particles. It should be noted that higher temperatures
are not feasible due to the limited temperature resistance of
the polyester textile. In conclusion, it can be stated that all
catalysts exhibit excellent CO conversion at the observed
temperatures. However, compared to the honeycomb and
foam ceramics, the polyester needle felt textile, in addition
to emission reduction, has the ability to filter particles from
the exhaust gas with high efficiency.
Considering the use of catalysts that achieve high pol-
lutant conversion even at lower temperatures, the scope of
application could expand from industry to households in
the future. Current indoor air purification systems tend to
focus on carbon capture and storage techniques [31]. The
use of catalytic pollutant reduction at ambient temperature
and slightly elevated levels, however, would be quite inter-
esting for health reasons. To assess the extent to which this
is achievable with conventional catalytic materials based on
Platinum, cerium oxide-modified catalysts were developed,
Fig. 8 Catalytic activity of Pt/Ce1/TiO2 (X), Pt/Ce2/TiO2 (Δ), Pt/Ce5/TiO2 (○), Pt/Ce10/TiO2 (). Conditions: 500 vppm CO, 16 vol% O2, 2
vol% H2O with N2 balance. Space velocity 30.000 h−1
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549Topics in Catalysis (2024) 67:539–550
showing promising results, as depicted in Fig.8. In com-
parison to the Pt/TiO2 foam catalyst (Fig.7c) at a space
velocity of 25,000 h−1, all modified materials exhibit better
conversion rates. Complete conversion can be achieved as
early as 65°C, despite the CO concentration in this experi-
ment being five times higher. With the exception of Pt/Ce2/
TiO2, a continuous increase in activity can be observed with
cerium loading. The previously described nonlinear cor-
relation between cerium loading and increased activity is
well-documented in the literature. It is presumed that the
interfacial area between cerium oxide and platinum is par-
ticularly responsible for increased activity at low tempera-
tures [32]. The most active catalyst, Pt/Ce10/TiO2, already
shows low conversions at 25°C and reaches 50% conversion
at 30°C. Complete conversion is observed at 55°C. Hence,
it is assumed that polyester needle felt filters equipped with
such low-temperature catalysts could also be used in indoor
settings (e.g., air purification of odors in the kitchen). Future
work may also investigate whether catalytic functionaliza-
tion by FSP can be extended to other types of textiles. Espe-
cially in the field of nitrogen oxide reduction on PTFE-based
textiles, the new coating method could present an interesting
alternative to previous preparation methods.
5 Conclusion
This study demonstrates that a flame spray pyrolysis setup
is suitable for coating an air-permeable polyester textile. At
a catalyst loading of 38g/l, the deposition of the particles
on the polyester needle felt textile substrate does not form
a continuous layer on the fibers, but rather a scattered and
uniform deposition. This is consistent with the fact that the
specific surface area of the textile is up to 25 times larger
than that of the ceramic substrates. Migration tests under
realistic conditions have shown that the adhesion of the cata-
lytic particles to the polyester textile is high and the catalytic
particles do not migrate. The results from the carbon monox-
ide conversion clearly indicate that the activity of the coated
textile and the ceramic substrates is similar. The conversion
rate for the coated textile is slightly higher, suggesting a
potential influence of the coating process on the catalytic
behavior. It has been demonstrated that complete conversion
of carbon monoxide can be achieved below 100°C under the
flow conditions typical of textile filter materials. Additional
experiments with ceria-modified Pt/TiO2 samples show that
high CO conversion can already be achieved at 40°C and
below. Low-temperature catalysis on a textile filter material
can be useful for various applications, including indoor pol-
lutant reduction.
Funding Open Access funding enabled and organized by Projekt
DEAL.
Data availability The data that support the findings of this study are
available from the corresponding author, Andreas Roppertz, upon rea-
sonable request.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
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